Summary Cryptosporidiosis is a common cause of severe, chronic diarrheal disease in immunocompromised patients such as those with HIV/AIDS. The only FDA approved drug for treatment of cryptosporidiosis, nitazoxanide, has limited effectiveness in immunocompromised patients. Despite the advent of HAART therapy, cryptosporidiosis still presents a major problem among patient populations where HIV diagnosis or anti-viral treatments are inadequate. As an enteric pathogen, Cryptosporidium interacts with the complex microbial community that constitutes the microbiome. Perturbations to the microbiota, such as through antibiotic treatment, are associated with increased susceptibility to infection in adult animals. Additionally, neonatal animals, which harbor an immature microbiota, are much more susceptible to Cryptosporidium infection than adults. One method by which the microbiota may influence susceptibility to Cryptosporidium infection is through the production of inhibitory small molecule metabolites. For example, high fecal indole levels were associated with lower susceptibility in a human challenge study of Cryptosporidium. Collectively, these findings raise the intriguing hypothesis that metabolites produced by the microbiota influence susceptibility to infection with Cryptosporidium. In preliminary studies, we have screened a library of bacterial metabolites that are abundant components of the normal adult microbiota for their ability to inhibit growth of C. parvum in vitro. We identified several classes of metabolites that are potent inhibitors of parasite growth at levels that are not toxic to host cells. The proposed studies will explore the mechanism(s) of inhibition by these microbial metabolites by determining which stage(s) of the parasite life cycle they target and whether they are static or cidal. We will also explore host cell signaling pathways that activate host defenses, as the potential mechanism by which these metabolites act. Finally, by employing animal models for cryptosporidiosis, we will test whether inhibitory metabolites can be used to treat infection in vivo. If successful, these studies may establish a new paradigm for treating persistent cryptosporidiosis in immunocompromised patients.